Surgically implanted systems, such as fixation devices and apparatuses, are commonly used to correct a variety of back structure problems, including those that occur as a result of trauma or improper development during growth. Generally, these systems correct such problems by providing a desired corrective spatial relationship between vertebral bodies. A typical spinal fixation system generally comprises a support rod or system of support rods that are secured along at least a portion of the spinal column intended to be immobilized by bone screws or hooks or other bone engaging components. Particular systems may include one or more fixation rods that are coupled to adjacent vertebra by attaching the rods to various anchoring devices, such as hooks, bolts, wires, or screws. Such bone anchors or devices may be directly connected to the support rods or may be connected indirectly by using medial/lateral connectors or other similar components. The bone screws, bone hooks, medial/lateral connectors, and/or related items that function to anchor the support rods to the bones are often collectively referred to as bone engaging hardware or implants.
Bone anchors such as screws and hooks are commonly utilized to facilitate segmental attachment of connective structures to the posterior surfaces of the spinal laminae. In a basic spinal fixation system, bone screws have a rod receiving opening extending through a head portion of each bone screw. The bone screws are typically secured through the pedicles and into the vertebral bodies at desired locations and a support rod is then extended through the opening in each bone screw. In particular, in order to accommodate connection to a spinal rod, many of these bone anchors are open-ended at an end distal from the end that is secured to a vertebra and have a yoke with a pair of upstanding arms that can receive a spinal rod in a channel formed between the arms. Because the rod connection portion of such bone anchors are open-ended, some type of fastener must be used in order to capture the rod or other structure as it is received within the open end of the anchor.
In order in order to fix the translational and rotational relationship of a support rod within the openings, various fastening techniques and devices can further be used to facilitate the securing of a spinal rod or rods to bone anchors of a spinal fixation system. For example, in one typical spinal fixation system, bone screws that have a rod receiving slot or opening in a head portion of the screw are implanted in predetermined vertebrae of the spine (adjacent vertebra, for example) and a spinal rod is then extended through the slot opening in each bone screw. The bones screws are then connected to the spinal rod by a setscrew or nut that engages the rod through or over a wall of the screw head. Tightening the setscrew or nut causes the spinal rod to be forced or clamped within the head of the bone screw thereby providing a holding force that attaches the spinal rod to the bone screw. The application of a pre-specified torque to the screw or nut provides a rigid construct for indefinite duration.
The fixation rods for a particular spinal fixation system are chosen according to a particular implantation site, and once installed, the fixation system holds the vertebral bodies in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time. For a spinal alignment correction with any of these systems that use a rod, the shapes of the support rods are utilized as the means to define and maintain the desired spinal curvature or vertebral alignment. Each rod is designed or selected to support a particular spine in a desired manner or to exert the desired corrective or stabilizing forces to the spine. Thus, each support rod can be bent or formed to a predetermined contour prior to positioning it in the rod-receiving opening of the bone screw. Alternatively, the support rods may be bent during the surgical procedure to accommodate the spinal correction or stabilization needed for each individual patient.
Other fixation systems have been developed that use medial/lateral connectors in association with bone screws to secure the support rods to the vertebra. The bone screws used in these systems typically include a threaded stud extending from the screw heads. The medial/lateral connectors include an arm and a head, and a rod receiving opening that extends through the head for connection to a support rod with a setscrew or other locking device. The arm of the connector includes an opening, such as a hole or slot that can receive the threaded stud of a bone screw. A fastener can then be used to attach the bone screw to the medial/lateral connector. This type of system is utilized in correcting spinal structural deformities or abnormalities is the same general manner as the fixation system above described wherein the support rods are shaped to define and maintain a desired spinal alignment.
With any of these systems that utilize a bone screw, it is common for the screw to pivot or otherwise move in at least one direction to achieve variable angular positions relative to the rod. In some cases, the screws are provided with the capability to move in three dimensions with respect to the rod in order to provide additional flexibility for positioning of the bone screw relative to the rod. These types of screws are often referred to as poly-axial or multi-axial screws.
Several types of multi-axial bone screw mechanisms have been developed and are available in the market. Generally, these are of two types that can be referred to as top-loaded and bottom-loaded systems. In top-loaded systems, a bone screw is assembled into a coupling member from the top, with the threaded portion inserted first and through the coupling member until the bone screw head engages the bottom portion of the cavity inside the coupling member. One disadvantage of these systems is that since the threaded portion of the bone screw must go through the coupling member, the dimensions of the coupling member must be able to accommodate the largest thread diameter of the bone screw. Typically, this results in a relatively large and bulky coupling member in a system that is desirably as small as possible.
In bottom-loaded systems, the bone screw is assembled into a coupling member from the bottom, which typically allows for a smaller coupling member since the screw member does not pass through the coupling member during assembly. However, one difficulty with bottom-loaded systems is preventing the head from being pushed out of the coupling member once it is inserted into the coupling member cavity. One design, which is disclosed in U.S. Pat. No. 6,660,004 to Barker et al., makes use of an internal retaining ring deployed inside a groove at the bottom of the coupling member. However, this design is limited in the amount of angulation between the bone screw and the coupling member. In order to maintain a desired level of strength for the grooved portion of the coupling member, a certain amount of material must be left at the bottom of the coupling member in order to support the retaining ring. This extra material impinges on the neck connecting the threaded portion to the head of the bone screw. In order to increase the angulation, the neck diameter must be reduced, which could weaken the bone screw. It is thus desirable to provide a bone screw system that allows for a higher degree of angulation while maintaining a sufficiently large neck diameter for strength of the screw.